How to pixelate a texture in Cycles?
This is done with rounding the mapping vectors.

The resolution for X and Y can be set with value nodes.
It will work for any texture (for 3D texture just copy the rounding nodes into the Z channel) and you have separate controls for X and Y resolutions.

If you also surround the Multi-Round-Divide nodes with Add and Subtract you can change the center point of effect (add and subtract the same number..)